Secondary batteries have been widely spread not only as power sources of portable devices such as mobile phones, digital cameras and laptop computers but also as vehicle or household power sources. In particular, a lightweight lithium ion secondary battery with a high-energy density is an energy storage device that is indispensable for our life. The lithium ion battery can be roughly categorized as a wound type battery or a laminated type battery. An electrode element of the wound-type battery has a structure in which a long positive electrode sheet and a long negative electrode sheet are wound multiple times in a state of being overlapped with separators respectively interposed therebetween. An electrode element of the laminated-type battery has a structure in which positive electrode sheets and negative electrode sheets are laminated alternately and repeatedly with separators respectively interposed therebetween. The positive electrode sheet and the negative electrode sheet each include an application portion where an active material layer (including a case of a mixture agent including a binding agent, a conductive material and the like as well as an active material) is formed on a current collector, and a non-application portion where the active material layer is not formed for the connection with an electrode terminal.
In each of the wound-type secondary battery and the laminated-type secondary battery, together with an electrolyte, the electrode element is accommodated and sealed in an outer container (outer case). Then, one end of a positive electrode terminal is electrically connected with the non-application portion of the positive electrode sheet while the other end is led out of the outer container, and one end of a negative electrode terminal is electrically connected with the non-application portion of the negative electrode sheet while the other end is led out of the outer container. With yearly improvements in battery technology, the trend is for annual increase in the capacity of the secondary batteries. What this means is that, should a short circuit occur, the amount of generated heat will increase which, in turn, increases safety risks. Therefore, measures to improve battery safety become more and more important.
FIG. 1 is a cross-section view that shows an example of a safety measure that is implemented to prevent the occurrence of a short circuit in the secondary battery. In FIG. 1, insulating tape 400 covers border portion 4 between an application portion where active material layer 200 is formed on current collector 300 and a non-application portion where active material layer 200 is not formed on current collector 300, and thereby, a short circuit is prevented.
Further, Patent Document 1 proposes a technology that uses a flexible insulating member for the border portion between the application portion and the non-application portion in the wound-type battery and can thus prevent damage to the insulating member when the electrodes are wound.
Moreover, Patent Document 2 discloses a technology that discharges an insulating member from an ink-jet coating device and attaches the insulating member to the border portion between the application portion and the non-application portion.